Major objectives in developing improved chemical processes typically are to use cheaper raw materials, to reduce energy demand, capital investment and operating costs. One application area that offers a large societal as well as economic benefit is the application of energy efficient, ultra low emission fuel cells in a “hydrogen economy.” Use of fuel cells range from the replacement of conventional internal combustion engines for transportation to displacement of batteries where long periods between recharging are required to distribute power production in residential homes and light industry. One of the challenges to making these fuel cell applications a reality relates to providing an inexpensive process for “storing” the hydrogen fuel used for these applications.
Convenient processes for storing hydrogen fuel relate to using methanol or dimethyl ether. With methanol, hydrogen can be released from the methanol molecule by a simple, low temperature reforming reaction to produce synthesis gas containing hydrogen. Dimethyl ether can be similarly reformed to produce hydrogen by reacting it with water.
The current costs of methanol and dimethyl ether as “fuel” compared to other hydrocarbons are high. It would thus be advantageous to develop a technology that could lower the cost of methanol and dimethyl ether to “fuel” cost levels. This could open up the use of methanol and dimethyl ether as viable fuels for fuel cells.
Methanol is typically manufactured by the hydrogenation of carbon monoxide over a copper-based catalyst, e.g., Cu/ZnO/Al2O3, in a fixed bed reactor. This technology has remained unchanged for over 30 years. Average catalyst life using this technology is about four years. Since the reaction is exothermic, there is a tradeoff between reaction kinetics and reaction thermodynamics. The reaction rate is greater at higher temperatures, whereas equilibrium is favored at lower temperatures. Operating at high temperatures can increase the rate of catalyst deactivation and produce undesired side products such as ketones that can form azeotropes, making product separation more difficult.
It is known that the intrinsic reaction rate for methanol synthesis is faster than the heat transfer rate between the reaction vessel and the reaction environment. This has the effect of limiting methanol production. Although theoretical kinetics suggest contact times on the order of milliseconds or tens or hundreds of milliseconds can be achieved, the slow rate of heat transfer typically necessitates contact times of seconds to minutes. It would thus be advantageous to develop a process that could utilize very short contact times on the order of milliseconds or tens or hundreds of milliseconds. This could dramatically increase the productivity of the reactor, that is, this could dramatically increase the throughput of the reactor per unit volume of the reactor. The inventive process provides such an advantage.
Dimethyl ether is commonly produced by dehydration of methanol over a dehydration catalyst. The reaction is exothermic and is equilibrium limited. Like methanol synthesis, the rate of the dimethyl ether synthesis reaction is limited by the ability to remove heat from the reactor. Improved heat removal could dramatically increase the reactor throughput of a dimethyl ether synthesis reactor. The inventive process provides such an advantage.
Dimethyl ether can also be produced directly from synthesis gas by a process that integrates methanol synthesis and dehydration into a single reactor and uses a combined methanol synthesis catalyst and dehydration catalyst. This direct synthesis process is exothermic and equilibrium limited. Direct dimethyl ether synthesis from synthesis gas is limited by the ability to remove heat from the reactor. Direct dimethyl ether synthesis can also be greatly improved by operation with very short contact times and high heat removal. The inventive process provides such an advantage.